Coxa Plana

Coxa plana is a descriptive term referring to the flattening and deformity of the femoral head that develops as a consequence of avascular necrosis of the proximal femur. The term derives from Latin, with coxa meaning hip and plana meaning flat, accurately describing the characteristic radiographic and anatomical appearance of the condition. Rather than maintaining its normal spherical shape, the femoral head becomes progressively flattened, irregular, and deformed as necrotic bone collapses under the mechanical stresses of weight bearing.

In dogs, coxa plana most commonly develops as the end-stage manifestation of Legg-Calve-Perthes disease, also known as avascular or aseptic necrosis of the femoral head. During the active phase of Legg-Calve-Perthes disease, the blood supply to the femoral head is disrupted, leading to death of the bone tissue within the femoral head. As the body attempts to repair the damaged bone through a process of resorption and revascularization, the structurally weakened femoral head deforms under normal loading forces, resulting in the characteristic flattened appearance.

The deformation that defines coxa plana is not merely cosmetic but has profound functional consequences for the hip joint. The normally congruent ball-and-socket relationship between the spherical femoral head and the cup-shaped acetabulum is disrupted, leading to abnormal joint mechanics, altered weight distribution across the articular surfaces, and progressive degenerative joint disease. The flattened femoral head no longer articulates smoothly within the acetabulum, producing pain, limited range of motion, and lameness.

Coxa plana predominantly affects small and miniature breed dogs, reflecting the breed predisposition of the underlying Legg-Calve-Perthes disease. The condition typically becomes apparent during the juvenile to young adult period, usually between four and twelve months of age, as the femoral head undergoes the sequential processes of necrosis, fragmentation, and deformed remodeling. Understanding coxa plana as a consequence of the avascular necrosis process is essential for appreciating the rationale behind treatment approaches and setting appropriate expectations for outcomes.

Coxa plana cannot be fully understood without appreciating its relationship to Legg-Calve-Perthes disease, as it represents one phase in the natural progression of this avascular necrotic process. Legg-Calve-Perthes disease begins with an interruption of the blood supply to the femoral head epiphysis, though the exact mechanism that initiates this vascular compromise remains incompletely understood. Proposed mechanisms include intravascular coagulation, increased intra-articular pressure compromising the retinacular vessels, and congenital vascular anomalies in the proximal femur.

The progression of Legg-Calve-Perthes disease through its various stages follows a predictable pattern that has been well characterized in both human and veterinary medicine. The initial stage of avascular necrosis is followed by a fragmentation stage in which the devitalized bone is resorbed and the structural integrity of the femoral head is progressively compromised. The reossification stage follows, during which new bone is laid down within the femoral head, and it is during this reparative phase that the shape of the femoral head is determined by the balance between biological repair and mechanical loading.

Coxa plana develops when the mechanical forces acting on the femoral head during the fragmentation and early reossification stages exceed the structural capacity of the weakened bone, causing it to deform plastically. The degree of deformation depends on several factors, including the extent of the initial necrosis, the rate of biological repair, the magnitude and direction of mechanical loading during the vulnerable period, and the degree of containment provided by the acetabulum. In cases where a large proportion of the femoral head is involved or where the repair process is slow, the resulting coxa plana is more severe.

The final shape of the femoral head after remodeling is complete determines the long-term functional outcome. Mild cases of coxa plana may retain sufficient sphericity to maintain reasonable joint congruency and function, while severe cases result in a markedly flattened and irregular femoral head that is grossly incongruent with the acetabulum. Secondary degenerative changes, including osteophyte formation, joint capsule thickening, and articular cartilage degeneration, develop progressively in response to the altered joint mechanics.

Coxa plana, through its association with Legg-Calve-Perthes disease, predominantly affects small and toy breed dogs, with certain breeds showing a markedly higher prevalence than others. Yorkshire Terriers are among the most commonly affected breeds, with a well-documented genetic predisposition to avascular necrosis of the femoral head. Studies have demonstrated that the condition occurs with sufficient frequency in Yorkshire Terriers to suggest a hereditary basis, likely involving an autosomal recessive mode of inheritance with variable penetrance.

Miniature Pinschers, Toy Poodles, and West Highland White Terriers represent other breeds with elevated risk for developing Legg-Calve-Perthes disease and its consequent coxa plana deformity. Chihuahuas, Pomeranians, Cairn Terriers, Maltese, and Manchester Terriers are also recognized as predisposed breeds. The common thread among these breeds is their small body size, typically under ten kilograms, though the precise relationship between small stature and susceptibility to femoral head avascular necrosis is not fully elucidated.

The breed predisposition strongly suggests a genetic component to the disease, and research has identified chromosomal regions associated with Legg-Calve-Perthes disease in some breeds. However, the condition does not follow a simple Mendelian inheritance pattern, and environmental factors likely modulate the expression of genetic susceptibility. Hormonal influences have been proposed as contributing factors, as the condition typically manifests during the rapid growth phase when hormonal activity is high and the demands on the developing vascular supply to the femoral head are greatest.

While the condition is overwhelmingly a disease of small breeds, rare cases have been reported in medium and larger breed dogs. In these atypical presentations, the underlying mechanisms may differ from those in small breeds, and other causes of femoral head avascular necrosis, such as trauma to the blood supply or corticosteroid administration, should be considered. The vast majority of clinical cases encountered in veterinary practice, however, involve the characteristic small breed patient presenting during the juvenile growth period.

The clinical presentation of coxa plana reflects the progressive deterioration of hip joint function as the femoral head deforms and degenerative changes develop. The most prominent clinical sign is hindlimb lameness, which may begin as subtle and intermittent before progressing to persistent and severe. The lameness is typically weight-bearing, meaning the dog can place the foot on the ground but shifts weight away from the affected limb, resulting in an asymmetric gait with a shortened stride on the affected side.

Pain is a consistent feature and can be elicited on manipulation of the hip joint during physical examination. Extension and internal rotation of the hip are particularly painful maneuvers, and dogs may vocalize, attempt to bite, or exhibit marked muscle guarding when these movements are performed. Crepitus, a grinding or crackling sensation palpable during hip manipulation, develops as the articular surfaces become irregular and the normal smooth gliding motion of the joint is disrupted by the deformed femoral head and secondary osteoarthritic changes.

Muscle atrophy of the affected hindlimb develops progressively as the dog preferentially unloads the painful limb. The muscles of the thigh, particularly the quadriceps and gluteal groups, become noticeably smaller compared to the opposite side. This muscle wasting can be detected by comparing the circumference of the thighs and by visual inspection from behind the standing dog. In bilateral cases, which occur in approximately ten to fifteen percent of affected dogs, the muscle atrophy affects both hindlimbs and may be more difficult to appreciate without careful examination.

As the condition progresses, range of motion in the affected hip becomes increasingly restricted. The normal arc of hip flexion and extension is reduced, and the dog may adopt compensatory gait patterns to minimize hip motion, including a stiff-legged walking style or a bunny-hopping gait in which both hindlimbs move together. Some dogs become reluctant to jump, climb stairs, or engage in active play, and in severe cases, the affected limb may be carried or used only minimally during locomotion.

Diagnosis of coxa plana is based on the combination of clinical findings and radiographic imaging. The signalment of a young, small-breed dog presenting with progressive hindlimb lameness and hip pain immediately raises suspicion for Legg-Calve-Perthes disease and its sequelae. Physical examination findings of hip pain on manipulation, decreased range of motion, crepitus, and thigh muscle atrophy further support the clinical suspicion and guide subsequent diagnostic imaging.

Radiography is the primary diagnostic tool and typically provides definitive diagnosis. Radiographic findings in coxa plana include a flattened and irregularly shaped femoral head that has lost its normal spherical contour. The femoral head may appear widened, fragmented, or moth-eaten depending on the stage of the disease process. The femoral neck may be shortened or thickened, and the acetabulum may show secondary remodeling changes including sclerosis and osteophyte formation. Widening of the joint space on the affected side compared to the normal hip is often evident, reflecting the loss of joint congruency.

Radiographic staging systems developed for Legg-Calve-Perthes disease help characterize the phase of the process and the severity of femoral head deformation. Early stages may show increased density or lucency within the femoral head without significant shape change, while advanced stages demonstrate the characteristic flattening, fragmentation, and remodeling that define coxa plana. Both hips should always be radiographed, as bilateral involvement occurs in a meaningful proportion of cases and the contralateral hip should be assessed for early changes even if clinically unaffected.

Advanced imaging modalities including computed tomography and magnetic resonance imaging can provide additional diagnostic information in complex or equivocal cases. CT offers superior bone detail and three-dimensional assessment of the femoral head deformity, while MRI can detect early avascular changes in the femoral head before radiographic changes become apparent. However, in the vast majority of clinical cases, standard radiography is sufficient for diagnosis, and advanced imaging is reserved for surgical planning or when the diagnosis remains uncertain after radiographic evaluation.

Conservative management of coxa plana may be attempted in dogs with mild clinical signs and early-stage disease, though it is important to recognize that conservative treatment does not reverse the femoral head deformity and is unlikely to prevent progressive degenerative changes. The goals of conservative management are to control pain, maintain function, and delay or potentially avoid the need for surgical intervention. This approach is most appropriate for dogs with mild lameness, minimal femoral head deformation, and owners who wish to explore nonsurgical options before committing to surgery.

Pain management forms the foundation of conservative treatment and typically involves nonsteroidal anti-inflammatory drugs administered under veterinary supervision. These medications reduce inflammation within the joint and provide analgesic relief, improving the dog's comfort and willingness to use the affected limb. Long-term use of anti-inflammatory medications requires regular monitoring of liver and kidney function, and the lowest effective dose should be used to minimize the risk of adverse effects.

Strict exercise restriction during the active phase of the disease is recommended to reduce mechanical loading on the weakened femoral head and minimize further deformation. Controlled leash walking on level surfaces replaces free running, jumping, and rough play. The duration and intensity of exercise should be tailored to the dog's comfort level, with activity reduced when lameness worsens and cautiously increased during periods of improvement. Weight management is essential, as excess body weight increases the forces transmitted through the affected hip joint.

Physical rehabilitation modalities can complement medical management and help maintain muscle mass and joint function. Passive range-of-motion exercises preserve joint mobility and prevent contracture formation. Therapeutic exercises targeting hindlimb strength help compensate for the mechanical disadvantage created by the deformed hip. Hydrotherapy, particularly underwater treadmill walking, allows active muscle engagement with reduced joint loading. Despite these measures, the majority of dogs with significant coxa plana eventually require surgical intervention, as conservative management typically provides only temporary improvement.

Femoral head and neck ostectomy, commonly abbreviated as FHO, is the most frequently performed surgical treatment for coxa plana in small breed dogs and is considered the standard of care for cases that fail to respond to conservative management. The procedure involves surgical removal of the deformed femoral head and a portion of the femoral neck, eliminating the painful bone-on-bone contact that is the primary source of discomfort. In the weeks following surgery, a fibrous pseudoarthrosis forms between the femoral shaft and the acetabulum, creating a functional but non-anatomic articulation.

The FHO procedure is well established in veterinary orthopedic surgery and has been performed for decades with consistently favorable outcomes in small breed dogs. The surgery is technically straightforward compared to total hip replacement and does not require specialized implants or equipment. This makes FHO accessible at most veterinary surgical practices and considerably less expensive than hip replacement. The procedure can be performed through various surgical approaches, with the craniolateral approach to the hip being most commonly employed.

Postoperative management is critical to achieving the best possible functional outcome. Early and aggressive physical rehabilitation encourages the formation of a well-organized fibrous pseudoarthrosis and prevents the muscle atrophy and limb disuse that can compromise recovery. Dogs should begin gentle passive range-of-motion exercises within days of surgery and be encouraged to bear weight on the operated limb as soon as comfort allows. Controlled leash walking is introduced early and gradually increased in duration and intensity over the following weeks.

The functional outcome following FHO in small breed dogs is generally very good, with the majority of dogs returning to comfortable, active lives without significant lameness. The small body weight of the typically affected breeds is a significant advantage, as the fibrous pseudoarthrosis is subjected to lower mechanical demands than it would be in a larger dog. Most owners report that their dogs return to normal or near-normal activity levels, including running, playing, and jumping, within two to three months of surgery, though full recovery and optimal function may continue to improve for up to six months.

Total hip replacement represents an alternative surgical option for dogs with coxa plana and offers the theoretical advantage of restoring normal joint anatomy and biomechanics through the implantation of prosthetic components. In this procedure, the deformed femoral head is replaced with a prosthetic femoral head and stem, and the acetabulum is resurfaced with a prosthetic cup, recreating the ball-and-socket articulation in prosthetic form. The result is a mechanically superior reconstruction compared to the fibrous pseudoarthrosis created by FHO.

Micro total hip replacement systems have been developed specifically for small and toy breed dogs, addressing the technical challenges posed by the diminutive size of the skeleton in these patients. These miniaturized implant systems allow total hip replacement to be performed in dogs as small as two to three kilograms, though the surgery requires specialized training, equipment, and implants that are not available at all veterinary facilities. The cost of total hip replacement is substantially higher than FHO, reflecting the complexity of the procedure and the expense of the implant components.

The outcomes of total hip replacement in small breed dogs with coxa plana are generally excellent when performed by experienced surgeons at specialized referral centers. The prosthetic joint provides normal range of motion, eliminates pain, and allows full return to activity without the biomechanical compromises inherent in the FHO pseudoarthrosis. However, total hip replacement carries risks including prosthetic luxation, implant loosening, infection, and femoral fracture that must be weighed against the benefits when choosing between surgical options.

The decision between FHO and total hip replacement for an individual dog with coxa plana involves consideration of multiple factors including the dog's size, activity level, the severity of the condition, the availability of specialized surgical expertise, the owner's financial resources, and the expected functional demands on the operated limb. For most small breed dogs, FHO provides excellent functional outcomes and remains the most commonly chosen surgical option. Total hip replacement may be preferred when optimal biomechanical restoration is desired, when bilateral disease requires the best possible function from each hip, or when the dog is active in demanding activities.

Rehabilitation following surgical treatment of coxa plana is essential for achieving the best possible functional outcome and represents a critical phase of treatment that requires active owner participation and compliance. The rehabilitation program should begin immediately after surgery and continue for several months, progressing through stages of increasing activity and intensity as healing and strength permit. A structured rehabilitation protocol guided by a veterinary rehabilitation professional optimizes recovery and helps prevent complications.

The early postoperative phase, encompassing the first two to four weeks after surgery, focuses on pain management, prevention of complications, and initiation of gentle therapeutic exercises. Passive range-of-motion exercises are performed several times daily to maintain joint flexibility and promote the organization of the developing pseudoarthrosis in FHO patients. Ice therapy applied to the surgical site helps control postoperative swelling and provides analgesic benefit. Controlled, short-duration leash walks encourage early weight bearing and prevent the deconditioning that accompanies prolonged inactivity.

The intermediate rehabilitation phase, spanning approximately four to eight weeks postoperatively, involves progressive increases in exercise duration and the introduction of active strengthening exercises. Leash walk duration is gradually extended, and varied terrain is introduced to encourage active hip flexion and extension. Therapeutic exercises such as sit-to-stand repetitions, gentle incline walking, and cavaletti pole walking build hindlimb strength and improve neuromuscular coordination. Hydrotherapy sessions, when available, provide excellent low-impact exercise during this phase.

The advanced rehabilitation phase extends from eight weeks through full recovery and focuses on building endurance, strength, and agility. Off-leash activity is gradually reintroduced in safe, controlled environments. Jogging, hiking, and play are resumed based on the dog's comfort and functional capability. By twelve to sixteen weeks postoperatively, most small breed dogs have achieved functional recovery and can return to normal household activities. Continued attention to weight management, regular moderate exercise, and monitoring for any signs of discomfort help maintain long-term joint health and function.

The long-term outlook for dogs treated for coxa plana is generally favorable, particularly in small breed dogs that have undergone successful surgical treatment. Dogs that recover from FHO typically maintain good function for the remainder of their lives, with the fibrous pseudoarthrosis providing adequate support for their lightweight frames. Some degree of limb shortening and altered gait mechanics is expected following FHO, but these changes are usually cosmetically subtle and functionally insignificant in small breed dogs.

Long-term complications following surgical treatment are uncommon but can include chronic pain at the surgical site, progressive muscle atrophy if the limb is not adequately used, and development of degenerative changes in the contralateral hip. Regular veterinary monitoring, including periodic orthopedic examinations and radiographs when indicated, helps detect any developing problems early. Maintaining lean body condition throughout the dog's life reduces mechanical stress on the operated hip and the contralateral limb, which may bear increased load to compensate.

Prevention of coxa plana is intrinsically linked to the prevention of Legg-Calve-Perthes disease, which requires responsible breeding practices in predisposed breeds. Dogs that have been affected by Legg-Calve-Perthes disease or coxa plana should not be used for breeding, as the strong hereditary component means that their offspring are at increased risk of developing the same condition. Screening of breeding stock with hip radiographs can identify subclinical cases, though the juvenile onset of the disease means that screening must be performed at an appropriate age.

Breed-specific health registries and databases that track the occurrence of Legg-Calve-Perthes disease and related conditions provide valuable tools for breeders making informed mating decisions. Genetic research aimed at identifying the specific mutations responsible for the condition in predisposed breeds holds promise for the development of DNA-based screening tests that could more effectively reduce prevalence. Until such tests are available, careful selection of breeding stock based on family health history and radiographic screening remains the most effective approach to reducing the incidence of coxa plana in susceptible breeds.

Owners of predisposed breeds should be educated about the early signs of hindlimb lameness and hip discomfort so that veterinary evaluation can be sought promptly. Early diagnosis and intervention, before severe femoral head deformation develops, may allow for better preservation of femoral head shape through strict activity restriction during the critical period, though the evidence supporting the superiority of early conservative management over eventual surgical treatment remains limited in veterinary medicine.